132 bulletin: museum of comparative zoology. 



plateaus were dependent on recent sub-cycles or episodes of erosion, 

 rather than on ■structure, some indication of the esplanade ought still to 

 be apparent in the Kaibab section at an altitude corresponding with 

 that so well maintained further west ; but inasmuch as the uplifts of 

 the Kaibab are believed to be of much earlier date than the erosion of 

 the canyon, the esplanade ought to stand at a lower geological horizon 

 in the Kaibab than in the Kanab. No trace of an esplanade is, however, 

 to be recognized in the Kaibab ; the descent from plateau to river is 

 there accomplished by a general incline, broken only by the benches of 

 the Eed-wall and Tonto groups, as stated above. On the other hand, 

 the slopes of the Kaibab walls and the cliffs and esplanade of the Kanab 

 walls are easily explained if they are dependent on local structural con- 

 ditions ; and there is independent evidence that the appropriate condi- 

 tions actually occur. It is evident that if the Tonto shales that form 

 the slope between the Red-wall and the Tonto cliffs in the Kaibab should 

 become more resistant as they pass westward into the Kanab, their 

 face would steepen, and the Red-wall cliff would come to stand more 

 immediately over the Tonto; at the same time the Eed-wall platform 

 would broaden to form the esplanade. Observations are not lacking to 

 support this supposition. Gilbert noted that, at the entrance of Kanab 

 creek into the main river, the Tonto shales are of " firm texture " (a, 

 p. 70). Button says more explicitly : " In the Kanab division the 

 whole series of nearly 2,500 feet thickness [Red-wall and below] is won- 

 derfully massive, and the partings of the strata are comparatively few. 

 In the Kaibab the great 750-foot limestone is as solid as ever, but most 

 of the other members have become laminated much more minutely than in 

 the Kanab and Uinkaret, and are of more perishable texture" (c, p. 257). 

 As these values of resistance are precisely such as would undermine the 

 Red-wall cliff-makers in the Kaibab and support them further west, 

 a structural explanation for the esplanade seems to be sufficient. 



Eastward Fading of the Esplanade. — In the second place, if the east- 

 ward extension of the esplanade be traced on the topographical maps, it 

 gradually loses definition by decrease in its breadth and by increase in 

 the number of ravines that break its front ; and in the easternmost part 

 of the Kanab it can hardly be recognized. This is best seen by tracing 

 the southern side of the canyon, for on approaching the Kaibab. the 

 form of the northern canyon wall is much complicated by the displace- 

 ments that occur there, and bedded structure has not alone a full control. 

 A gradual change of form of this sort is an appropriate consequence of 

 the structural origin suggested for the esplanade. 



